It's been stated that you can't block when wiedling magic in one of your hands.
Where? I thought that was only with a one-handed weapon equipped with a spell.
As for parrying with dual-weapons I'm not sure. In a recent interview Todd said you could slash left and right (controlled) by using the direction stick/mouse and also mentioned there would be combos including being able to 'jump backwards', so I think they're trying to make the 'lighter' styles more dodge based. I'll try and find that interview now.
EDIT: http://www.gamesas.com/index.php?/topic/1214087-may-2011-gamerno-norwegian-magazine-interview/
"- How are sword fighting? Can you choose to swing your sword to the left or right?
- Yes, you choose the direction with the control stick. It is also possible to perform different combo attacks, jump back and stuff."